The preferred devices employed for separating a fine-grained solid with a particle size distribution ranging between 0 and max. 50 .mu.m into a fines fraction and a coarse fraction at a cut point of below about 10 .mu.m are hydrocyclones, in which the separation is achieved through the combined actions of centrifugal force, wall friction, and the drag force of a liquid exercised on the solid particles. Technical reasons related to the flow conditions in a hydrocyclone, however, make a sharp separation in the case of a defined particle size impossible, so that the overlap range, i.e. the particle size range which is present in both the fines fraction and the coarse fraction, is usually undesirably high.